The present invention relates to a floating offshore structure and more particularly a floating offshore structure of a semisubmersible type such as a drilling rig for crude oil production or a marine leisure facility which is so designed and constructed to decrease its heaving motion in waves.
A floating structure as shown in FIG. 1 is well known in the art.
In FIG. 1, reference numeral 11 designates water surface; 12, column; 13, a constricted or reduced-diameter part; 14, a floater; and 15, a deck. The following relation must be satisfied: EQU d.sub.3 &gt;d.sub.1 &gt;d.sub.2
wherein
d.sub.1 : diameter of the column 12;
d.sub.2 : diameter of the reduced-diameter part 13; and
d.sub.3 : diameter of the floater 14.
The conventional floating structure with the reduced-diameter part 13 described above has such a shape that it will receive no vertical wave-induced force at two wave periods, that is, at a considerably short wave period and at a relatively long wave period, whereby the heaving motion of the floating structure is decreased over a wide range of wave period.
However, when the upper column 12 is increased in diameter d.sub.1 to provide the reduced-diameter part 13, there arises the problem that heave resonant period becomes shorter since the column 12 has the larger diameter d.sub.1 at the cross section thereof defined by the water surface 11 and extends, without changing its diameter, up to the deck 15.
The present invention has for its object, therefore, to overcome the above and other problems encountered in the conventional offshore structures.